In a spread spectrum communication system, symbols or packets are spread and scrambled prior to transmission. While the spreading code used to spread an in-phase (I) component of the transmitted signal s(t) may sometimes be identical to that used for the quadrature (Q) component, generally the scrambling codes differ. The end result is that the I and Q components are identified by different component spreading codes cI and cQ. A receiver generally acquires at least the initial spreading codes from one or more synchronization channels, which also provide the receiver with phase/frequency to be used for demodulating the received signal r(t). However, the signal is subject to phase errors introduced in the transmission channel that are not accounted for by information provided over the synchronization channel.
Bi-BPSK is a modulation technique wherein each of the in-phase (I) and quadrature (Q) components of a received signal r(t) are treated as independent binary phase shift keyed (BPSK) signals. In general, each of the two BPSK signal components may be independently scaled and/or spread, generally using different component spreading codes cI and cQ or scaling factors. Though the data rates over the I and Q channels may differ at any given instant, the chip rate of each (the rate of the spreading sequences in a spread spectrum communication) is generally identical.
A Bi-BPSK constellation is shown in FIG. 1, wherein the constellation points are located at positions (−2, −1), (−2, 1), (2, −1) and (2, 1), and received points are grouped about the true constellation points. Where the I axis is horizontal representing a cosine function and the Q axis is vertical representing a sine function, phase error introduced in a transmission channel tends to rotate the vector defining the point relative to the I and Q axes. Phase error correction seeks to drive the I or Q value to zero by de-rotating the position vector of received points. This clusters received points more tightly about the actual constellation points, reducing error in resolving which of the constellation points the received point truly represents. Correcting for phase error clusters all of the received points more closely toward the constellation points, eliminating ambiguities that may be otherwise cause a received point to be wrongly decided. In the prior art, phase error correction is typically done on the Q channel, since it generally represents the stronger signal, provided the Q channel is spread more. To derotate the phase error, the prior art used a closed loop to drive the product of IdQ*sign(QdQ). Bi-BPSK often takes the approach of using a single carrier phase error term to resolve phase error in both the I and Q channels. That prior art approach is subject to higher phase jitter in the carrier loop when phase error varies over time, leading to unacceptably high symbol error rates in certain cases. The present invention includes a method and apparatus for reducing phase jitter in the carrier loop and for reducing the commensurate symbol error rate.